def is_prime(n):
 if n < 2:
return False
for i in range(2, int(n**0.5) + 1):
if n % i == 0:
return False
return True
def max_prime(num): 
if len(str(num)) != 4:
print(0)
return
digits = [int(d) for d in str(num)]
digits.sort(reverse=True)
for i in range(4, 0, -1):
for j in range(0, 4-i+1):
 num = int("".join(map(str, digits[j:j+i])))
if is_prime(num):
print(num)
return
print(0)
num = int(input())
max_prime(num)